(1) Field of the Invention
The present invention relates to an air/fuel ratio sensor, an air/fuel ratio detecting and controlling apparatus, and a method of detecting normal and abnormal conditions of the sensor, capable of accurately detecting and controlling air/fuel ratio.
(2) Description of the Prior Art
In recent years, feedback control of the air/fuel ratio engines has come into use in order to improve the operating characteristics and to lower fuel consumption.
Examples of air/fuel ratio sensors for use in such feedback control are disclosed for example in Japanese Patent Disclosure Nos. 59-67455 and 59-46305. Such air/fuel ratio sensors are generally referred to as being of oxygen-sensing type, in which an amount of oxygen which diffuses between electrodes expresses an oxygen concentration, with this amount of diffusing oxygen being detected as a diffusion current (pump current). In this way the oxygen concentration (air/fuel ratio) can be detected. The pump current flows through an oxygen sensor which is mounted within the exhaust path of the engine, to sense the concentration of oxygen in the exhaust gas, and the air/fuel ratio is detected from the value of this current. More specifically, oxygen molecules which are ionized by the action of a pump cathode are pumped through a solid electrolyte to a pump anode, and the resultant ion current (pump current) is used to detect the concentration of oxygen in the exhaust gas. The air/fuel ratio is correlated with this oxygen concentration.
With such a prior art type of air fuel ratio detection apparatus, insufficient means are available for counteracting the effects of deterioration of the air/fuel ratio sensor and changes in the sensor characteristics over a long period of time. As a result, as time elapses, the sensitivity of detection of the air/fuel ratio decreases and this can result in a lowering of the degree of control of the engine air/fuel ratio. For example, if an oxygen sensor as described above is used as the air/fuel ratio sensor, then due to deterioration of the pump anode over a long period of time, deterioration of the pumping action of the solid electrolyte, or a lowering of the diffusion factor of the diffusion layer, the correlation between the sensor output and the oxygen concentration in the exhaust gas will change. The detection accuracy will thereby be lowered. In addition, if carbon becomes deposited between the electrodes, leakage current will flow and this will also contribute to lowering of the detection accuracy.
FIG. 1 illustrates the changes which can thereby result in the output characteristic of an air/fuel ratio sensor, i.e. non-linearity of the characteristic, as indicated by the broken-line portion. In such a case, even if correction of the air/fuel ratio sensor is performed for the case of exhaust gas having a high concentration of oxygen, satisfactory correction overall will not be possible. As a result, it will not be possible to detect the air/fuel ratio accurately, so that it will not be possible to control the air/fuel ratio precisely to a desired target value.